This invention relates to N-phosphonomethylglycine (glyphosate) and more specifically, to a method of making concentrated monosodium iminodiacetic acid solutions for the preparation of N-phosphonomethylglycine.
N-phosphonomethylglycine (glyphosate) is an important broad spectrum herbicide. One conventional precursor to glyphosate is N-phosphonomethyliminodiacetic acid (PMIDA) having the following formula(I): EQU H.sub.2 O.sub.3 PCH.sub.2 N(CH.sub.2 COOH).sub.2
An early description of the preparation of above compound (I) is disclosed in U.S. Pat. No. 3,288,846 to Irani et al. In such process iminodiacetic acid (IDA) was employed as a starting material, prepared by hydrolysis of iminodiacetonitrile. Several process steps are required for the recovery and purification of IDA. The acid is then phosphonomethylated with phosphorous acid and formaldehyde in the presence of a mineral acid. In one embodiment, the addition of phosphorus trichloride to the aqueous reaction mixture provides, by hydrolysis, sufficient hydrogen chloride to form the hydrochloride salt of IDA while also providing the phosphorous acid for the phosphonomethylation reaction.
An improvement of the Irani et al process is disclosed in U.S. Pat. No. 4,724,103 to Gentlicore. According to this improvement the dialkali metal salt of IDA, preferably the disodium salt (DSIDA), is employed as the starting material. Upon reaction with a strong mineral acid, typically hydrochloric acid, the DSIDA is converted to the acid salt (IDA.cndot.HCl) and the alkali metal salt of the strong acid. The acid salt is then phosphonomethylated by reacting the strong acid salt with phosphorous acid and formaldhyde to provide compound (I) and an alkali metal salt. Water and caustic are added to the reaction mixture in an amount sufficient to dissolve the alkali metal salt and compound (I) is separated as a precipitate. This process eliminated numerous steps for the purification and recovery of IDA from the crude hydrolysate of IDAN thus offering a more economical route to (I). For a description of the numerous steps required to separate and purify IDA from the IDAN hydrolysate see British patent 1,575,469. The elimination of much capital equipment and processing steps will be appreciated by a comparison of the processes of Gentlicore and the British patent.
The process of Gentlicore requires that the DSIDA starting material be maintained at elevated temperatures during shipping and storage because this material crystallizes at temperatures below about 55.degree. C. The crystal form is highly insoluble and inconvenient to dissolve in water for use in the process for preparing compound (I). Accordingly, DSIDA must either be used immediately following its production or stored in a diluted form, or heated to maintain solubility. All such alternatives increase cost.
One attempt to avoid the problem associated with the use of DSIDA is disclosed in U.S. Pat. No. 5,312,972 to Cullen. Cullen discloses an alternative process for preparing compound (I) that involves reacting solutions of a dialkali metal salt of IDA with formaldehyde so as to form the dialkali metal salt of hydroxymethyliminodiacetic acid (HMIDA). Such precursor has the potential for increasing loads of by-product N-methyl iminodiacetic acid in the final product. The HMIDA can be subsequently reacted with a phosphorus source such as phosphorous acid to produce compound (I).
Although the above-mentioned methods achieve their intended purposes, they are limited in use. In particular, DSIDA must be diluted or kept hot to maintain solubility of the concentrate during transport or storage. Dilute solutions contain more liquid and are therefore more expensive to ship. In some instances it is impractical or costly to store and ship solutions while heating to a temperature range sufficient to maintain solubility and prevent crystallization. Using formaldehyde as in Cullen to maintain solubility in concentrated disodium iminodiacetic acid solutions is unacceptable for toxicity reasons.